A belt-type continuously variable transmission (henceforth referred to as CVT) is composed of a primary pulley at an input side, a secondary pulley at an output side, and a belt, wherein a torque of an engine is inputted to the input side, and the output side outputs a torque to road wheels, and the belt is wound between the primary pulley and the secondary pulley. Each of the primary pulley and the secondary pulley includes a fixed pulley and a movable pulley which form a V-shaped groove. Each movable pulley is biased toward the fixed pulley by a primary pulley pressure (henceforth referred also as primary pressure) or secondary pulley pressure (henceforth referred also as secondary pressure) produced from a line pressure as an original pressure. The belt is thus sandwiched by the pulleys, to transmit power between the primary pulley and the secondary pulley.
Incidentally, it is known that fuel efficiency is influenced significantly by height of the line pressure, because the line pressure is generated from a discharge pressure of an oil pump as an original pressure. Furthermore, if the line pressure is unnecessarily high, it becomes a factor for increasing friction at a rotating part or sliding part of the transmission. Accordingly, a technique has been proposed conventionally, which is configured to reduce the line pressure down to a required pulley pressure, and thereby reduce the discharge pressure of the oil pump and reduce the friction, and thereby enhance the fuel efficiency.
For example, a patent document 1 describes a line pressure control device configured to set a target line pressure to a higher value of a required primary pressure and a required secondary pressure, and thereby regulate the line pressure to a minimum required value, and thereby enhance the fuel efficiency effect. At a high side where the primary pressure is higher than the secondary pressure, this technique is further configured to prevent the line pressure control from being affected by errors of attachment of a shift actuator, and thereby prevent the line pressure from being corrected unnecessarily, and thereby suppress the fuel efficiency from being adversely affected by excessive increase of the line pressure, and suppress the occurrence of a failure to establish a target transmission ratio due to shortage of line pressure.
Incidentally, in patent document 1, the required secondary pressure is determined based on an actual transmission ratio and an input torque, and a feedback control is performed based on a deviation from an actual secondary pulley pressure sensed by a sensor. Namely, a pressure reducing valve, which is disposed in an oil passage connected to a secondary pulley chamber, is controlled by feedback control, to conform the actual secondary pulley pressure to the required secondary pressure.
The technique disclosed in patent document 1, which is configured to control the line pressure to equalize the line pressure with the required primary pressure, is effective especially when the transmission ratio is at the high side. In the entire region of transmission ratio, the friction can be reduced by controlling the line pressure to equalize the line pressure with the required secondary pressure also when the transmission ratio is at a low side. Namely, the control of the line pressure to conform the line pressure to the higher pulley pressure of the primary pressure and the secondary pressure, is effective for enhancing the fuel efficiency effect while ensuring the minimum required line pressure.
Incidentally, in general, in a CVT, the pulley pressure is controlled to a command value (i.e. pulley pressure command value) by feedback control. For implementing the feedback control, a pulley pressure sensor is provided to sense the pulley pressure. For example, when the line pressure is controlled to the secondary pressure with the transmission ratio at the low side, control of the line pressure to a secondary pressure command value is effective for enhancing the fuel efficiency effect. However, when no oil pressure sensor is provided to sense an actual line pressure, the actual line pressure cannot be obtained directly. The actual line pressure and actual secondary pressure are used to refer to quantities obtained by oil pressure sensors.
In view of the foregoing, it is effective to proactively control the line pressure command value to equalize the actual line pressure with the actual secondary pressure (referred to as pressure-equalizing control), and thereby conform a sensed value, which is obtained by a pulley pressure sensor (secondary pressure sensor) sensing the actual secondary pressure, to the actual line pressure, and control the sensed value of the secondary pressure sensor to the secondary pressure command value by feedback control.
If the pressure-equalizing control to equalize the line pressure with the secondary pressure is performed during feedback control of the secondary pressure, this may be implemented by reducing the line pressure command value from a current value. Namely, since an upper limit of the secondary pressure is restricted by the line pressure, the sensible secondary pressure is reduced with decrease of the line pressure by reducing the line pressure command value, and thereby reducing the line pressure. Accordingly, when the decrease of the actual secondary pressure is determined as the line pressure command value is reduced, it can be determined that the line pressure has been equalized with the secondary pressure.
Once the pressure-equalized condition is determined (pressure-equalization determination), the actual secondary pressure can be regarded as equal to the actual line pressure thereafter. Accordingly, after the pressure-equalization determination, it is possible to reflect a controlled quantity (i.e. feedback correction quantity), which is to be feedback-controlled to conform the actual secondary pressure to the target secondary pressure, on the control of the line pressure command value, and thereby adjust the secondary pressure to the target secondary pressure by this line pressure control while maintaining the line pressure and the secondary pressure to be equalized with each other.
The pressure-equalization determination causes a decrease of the secondary pressure, because the pressure-equalization determination is in response to decrease of the actual secondary pressure as the line pressure command value is reduced. Furthermore, in general, sensed value of the oil pressure sensor that senses the actual secondary pressure as a factor for the determination are unstable with (harmonic) oscillating components. Even if a low-pass filter is used to remove the oscillating components from the sensed value of the oil pressure sensor, it is impossible to determine the pressure-equalized condition unless the actual secondary pressure is reduced by some quantity.
The decrease of the actual secondary pressure may cause the belt to slip. In order to prevent the slip, in consideration of the decrease of the secondary pressure required for the pressure-equalization determination, a control to proactively raise the secondary pressure by the quantity of decrease of the secondary pressure is performed, immediately before the reduction of the line pressure command value. Namely, the pressure-equalizing control is implemented by: first increasing the secondary pressure command value by a predetermined quantity; and when the actual secondary pressure has increased by the predetermined quantity, reducing the line pressure command value; and when the actual secondary pressure has thereby decreased to the state before the secondary pressure command value is increased by the predetermined quantity, determining that pressure-equalized condition is determined; and thereafter controlling the line pressure to conform the actual secondary pressure to the secondary pressure command value with the actual secondary pressure regarded as the actual line pressure.
However, it has been found that when the pressure-equalizing control is performed in this way, the transmission ratio fluctuates unstably while the secondary pressure is reduced by lowering the line pressure command value. By consideration of a mechanism causing the phenomenon, the following can be assumed.
During the pressure-equalizing control, at the primary pulley side, the primary pressure is also feedback-controlled to conform the actual transmission ratio, which is obtained by the rotational speed of the primary pulley and the rotational speed of the secondary pulley sensed by respective rotation sensors, to the target transmission ratio. As the secondary pressure is reduced as described above, the pulley diameter of the secondary pulley changes in the decreasing direction, thereby changing the actual transmission ratio to the high side. In response to this change, the primary pressure command value is reduced to suppress the change of the actual transmission ratio to the high side. However, it is conceivable that since the feedback control of the primary pressure has a response delay, the change of the actual transmission ratio to the high side cannot be suppressed sufficiently, but the fluctuation of the actual transmission ratio becomes large.
The present invention is made in view of the foregoing problems. It is an object of the present invention to provide a continuously variable transmission control device and a continuously variable transmission control method which allow to enhance the fuel efficiency by controlling the line pressure to the minimum required value, and suppress the resulting occurrence of unstable fluctuation of the transmission ratio. In addition, it may be regarded as other objects of the present invention to produce advantageous effects which result from configurations of embodiments of the present invention described below and cannot be obtained by conventional technology.